1. Field of the Invention
The present invention relates to a photographic lens system, and more specifically, to a wide-angle photographic lens system optimum for a camera capable of exposing at a panoramic size format.
2. Description of the Prior Art
In recent years, there has been a demand for wider-angle lens systems for lens shutter cameras, more particularly for cameras capable of exposing at a panoramic size format.
As conventional wide-angle photographic lens systems, ones are known which are made compact by employing lens arrangements of a retro focus type frequently used for single-lens reflex cameras, of a symmetrical type such as a topogon, and of a telephoto type (a type where, with respect to an aperture, an object side lens unit has a positive refractive power and an image side lens unit has a negative refractive power) such as the one disclosed in Japanese laid-open Patent Application No. S58-219509.
However, the retro focus type lens system is not suitable for reducing the total length of the lens system since the back focal length is large, while concerning the symmetrical type lens system such as the topogon, since it is difficult to correct field curvature in a compact lens system having a large diameter, it is difficult to realize a high performance. In the telephoto type lens system where the lens unit arranged in front of the aperture generally includes three lens elements consisting of a positive, a negative and a positive lenses (this type is called a triplet type), since the deterioration of image plane quality (that is, field curvature and astigmatism in the periphery of the image plane) cannot sufficiently be corrected by use of the lens unit arranged behind the aperture if the angle of view is increased, it is after all difficult to realize a high performance.